Electronic slip control

ABSTRACT

The outputs of two r.p.m. signal generators connected respectively to the driven and the undriven axles are fed to respective pulse shapers, the outputs of which are conducted to an analog divider that provides an output indicative of the slip between the two axles. This output operates a hydraulic control for raising and lowering the plow.

g Umted States Patent 1191 1111 3,776,322 Misch et al. 1 Dec. 4, 1973 ELECTRONIC SLIP CONTROL 3,617,099 2 1971 Sugiyama 324/161 x [75] Inventors: Wolfgang Misch, Stuttgart; Helmut Dmnann; Karl-Heinz Adler both of 3,560,854 2 1971 Moss et a]... 324/161 x Leonberg, all of Germany 3,500,190 3 1970 Michon 324/161 3,609,313 9 1971 Lucien.... 303 21 P ux [73] Assgnee' Robert Bosch Stuttgart 3,614,173 10/1971 Branson 303 21 P Germany [22] Filed: May 6, 1971 [21] Appl. No.: 140,690

[30] Foreign Application Priority Data References Cited UNITED STATES PATENTS l0/19 62 Buttenhoff 172/2 X Primary ExaminerStephen C. Pellegrino Attorney-Michael S. Striker 9 Claims, 5 Drawing Figures PMENTEDnEc M975 3.776.322

- sum 1 or 5 Fig. 7

3);? NVVENTORS! Wolfgang M I SCH He/mu/ 30/74 N N Karl- Heinz ADLER l-beir A 7" 7' ORNE y SHEET l UF 5 IAN E N TORS.

Mimi-Heinz 40416 ELECTRONIC SLIP CONTROL BACKGROUND OF THE INVENTION The invention relates to means for providing a signal indicative of the amount of slip between a driven axle and an undriven axle, and for measuring and/or controlling this slip. The invention is particularly suitable for measuring the slip between the driven axle and the undriven axle of a tractor, such as a tractor used for plowing.

Experiments have shown that a tractor operates most efficiently the least fuel for a given area of plowed land being used if the driven axle has a slip of from 9 to percent. In other words, the drive wheel must turn faster than is necessary to reach the desired forward speed. While carrying out the aforementioned experiments, the tractor operator had, in his view, a measuring instrument that showed the value of the slip. As soon as the slip exceeded l5 percent, he raised the plow by means of the hydraulic control, and so reduced the pulling force exerted by the tractor. When the slip fell below 9 percent, he again lowered the plow.

SUMMARY OF THE INVENTION The simplest arrangement for measuring the slip comprises a respective tachogenerator for the driven axle and the undriven axle, the tachogenerator providing an output signal proportional to the r.p.m. A quotient meter with crossed coils can be used to measure the slip. The alternating voltage output from the tachogenerators is rectified by respective diodes and fed to respective ones of the two coils of the quotient meter.

An object of the invention is a very simple arrangement that automatically enables the plow to be raised and lowered so as to maintain a desired amount of slip for the most efficient use of fuel in respect of the amount of the area plowed.

Essentially, the invention consists of a first r.p.m. signal generator for delivering a signal indicative of the r.p.m. of the driven axle, a second r.p.m. signal generator for delivering a signal indicative of the r.p.m. of the undriven axle, first and second pulse shapers connected to receive as input the respective outputs of the first and second signal generators, and an analog divider for providing as output a signal indicative of the slip between the driven and the undriven axles, the analog divider having two inputs, these two inputs being connected to receive as input the output of respective ones of the first and second pulse shapers.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an embodiment of the invention;

FIG. 2 is a wiring diagram of one embodiment of the analog divider;

FIG. 3 is a wiring diagram of a second embodiment of the analog divider;

FIG. 4 is a wiring diagram of one embodiment of the pulse shaper,

FIG. 5 is an illustration of one type of vehicle for which the present invention is intended.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, the tachogenerators 11 and 12 each comprise a toothed wheel made of soft iron and mounted, respectively, on the front and rear axles of a vehicle (see FIG. 5); the teeth of the wheel, when the latter turns, changing the magnetic flux in coils mounted in operative proximity to the respective toothed wheels and inducing therein respective alternating voltages, the frequencies of which are directly proportional to the speed of the toothed wheels. The output of each tachogenerator is conducted to a respective pulse shaper l3 and 13'. Each pulse shaper consists of an amplifier 14 or 15, a Schmitt-trigger 16 or 17, and a monostable multivibrator 18 or 19, all connected inseries. The output pulses of the two pulse shapers l3 and 13' are conducted to respective inputs 25 and 26 of an analog divider 20.

The analog divider 20 includes an operational amplifier 22 having two inputs 30 and 31 and an output 32. The second input 31 of the operational amplifier 22 is also the second input 26 of the analog divider 20. A feed-back connection 40 couples the input 30 of the operational amplifier 22 to the output 32. The feedback connection 40 includes the second input 28 and the input of a multiplier 21. The first input 27 of the multiplier 21 is also the first input 25 of the analog divider. An electro-hydraulic control 24 for raising and lowering the plow is connected to receive the output of the operational amplifier 22. If desired, there can be connected between the output 32 of the operational amplifier 22 and the input of the electro-hydraulic control 24 further amplifier 23, as shown in FIG. 1. A stabilized source of direct current 25 is connected to the pulse shapers l3 and 13' and to the analog divider 20 to provide power therefor. FIG. 5 illustrates very schematically one form of wheeled vehicle for which the invention can be used. In FIG. 5, the vehicle is a tractor having a ground-penetrating implement 92 supported on a pair of support arms 93 (only one illustrated). Conventional electrohydraulic lifting means 94, e.g. an hydraulic cylinder-and-piston arrangement, is provided and is operative for raising and lowering the groundpenetrating implement 92 in response to electrical control signals. The tractor has rear driven wheels 91 and front undriven wheels 90. Mounted on the front of undriven axle is a first toothed wheel of ferromagnetic material cooperating with a non-illustrated pick-up coil to form a first signal generator 11. Mounted on the rear or driven axle is a second toothed wheel cooperating with a non-illustrated pick-up coil to form a second signal generator 12. Each of these signal generators delivers a pulse train whose frequency is proportional to the speed of rotation of the respective axle.

FIG. 2 is the wiring diagram of the analog divider 20. The two inputs 25 and 26 are connected by the resistors 44 and 45 to the bases of respective transistors 41 and 42. The emitter of the first transistor 41 is directly connected to the positive rail 48. The collector of the transistor 41 is connected by two resistors 46 and 47 to the negative rail 49. A resistor 50 connects the junction between the resistors 46 and 47 to the base of a third transistor 43. The emitters of the transistors 42 and 43 are connected to a lead 67, which is connected by a resistor 52 to the positive rail and by a capacitor 51 and a resistor 53 to the negative rail 49. A resistor 54 con nects the collector of the transistor 42 to the feed-back connection 40. The collector of the transistor 43 is connected by a resistor 56 to the negative rail 49.

The end terminals of a trimming potentiometer 58 are connected by resistors 55 and 57 respectively to the collectors of the transistors 42 and 43. The movable tap of the trimming potentiometer 58 is connected by a capacitor 59 to the feedback connection 40, and is directly connected to the first input 30 of the operational amplifier 22. A setting resistor 60 connects the lead 67 to the second input 31 of the operational amplifier 22. The electrohydraulic control means 24 and the resistor 63 are connected in series between the positive rail 48 and the operational amplifier output 32. To supply electric power to the operational amplifier, the terminals 64 and 65 are respectively connected to the positive rail 48 and the negative rail 49. A capacitor 62 connected between the output 32 and a terminal 66 of the operational amplifier provides frequency compensation at the high frequencies.

FIG. 4 shows the pulse shaper 13, the amplifier stage 14, the Schmitt trigger l6, and the monostable multivibrator 18 being enclosed within dashed lines. The output signal of the tachogenerator 11 is conducted by a series connected capacitor 91 and resistor 92 to the base of an amplifying transistor 141. The base current of this transistor flows through the resistors 144, 143, and 142, this latter resistor being the collector resistor of the transistor. A capacitor 145 reduces the degenerative feedback from the collector to the base of the transistor 141.

The amplified signal on the collector of the transistor 141 is conducted by a resistor 146 to the Schmitt trigger 16, which comprises the two transistors 161 and 162 having a common emitter resistor 163. The collectors of these two transistors are connected by respective resistors 164 and 165 to the positive rail 48. The output signal of the trigger 16 is coupled by a capacitor 166, a voltage divider having the resistors 167 and 168, and by a diode 169 to the monostable multivibrator 18, which has the two transistors 181 and 182. These two transistors are mutually coupled resistively by a resistor 191 and capacitively by a variable RC network composed of the capacitor 189 and the variable resistor 190. The time constant of the RC network can be changed to vary the pulse length of the monostable multivibrator with a view to adapt the analog divider to different kinds of r.p.m. generators 11. The output signal of the pulse shaper 13 appears across the collector resistor 192 of the transistor 182.

The pulse shaper 13 and the analog divider 20 operate in the following manner. The approximately sin wave signal of the tachogenerator 11 is amplified by the stage 14. The Schmitt trigger l6 converts the amplified, approximately sin wave, signal into rectangular pulses of precisely defined amplitude. These rectangular pulses operate the monostable multivibrator 18, which produces rectangular pulses of which the pulse length is independent of the pulse repetition frequency. The amplitude of these pulses is also constant, and the pulse repetition frequency is proportional to the r.p.m. of the axle that turns the r.p.m. signal generator 11.

The circuit diagram of the pulse shaper 13' is exactly the same as that for the pulse shaper 13, although the capacitor 189 and the resistor 190, which constitute the timing network for the monostable multivibrator 18, must have different values when the driven wheel of the tractor has a larger diameter than the undriven wheel. Consequently, it is possible to use the same kind of tachogenerator for each axle, even though the two wheels have different diameters. it is not necessary to use tachogenerators with toothed wheels having different numbers of teeth in dependence on the wheel diameters.

During the course of the succeeding explanation, the following abbreviations will be used: n denotes the r.p.m. of the driven axle, n,, the r.p.m. of the undriven axle, u the output signal of the second pulse shaper 13, u,, the output signal of the first pulse shaper 13, and u the output signal of the operational amplifier 22. FIG. 2 shows that the analog divider 20 is composed of analog computer components, namely a multiplier (transistor 42) connected in the feed-back circuit 40 of the operational amplifier 22. Consequently, the output signals u and u, are multiplied together, and the resulting product added to the value 14,, at the summation junction 30. In the active range of the circuit, the summation junction 30 is always at zero potential, so that there results From the preceding, it is apparent that 14 and u should be out of phase. The first transistor 41 inverts the phase of the signal u,,.

The second transistor 42 operates as a time-division multiplier. The base 27, which is the first input of the multiplier, is fed rectangular pulses having a constant amplitude and pulse length, but a variable pulse repetition frequency. if this input voltage is integrated, the time integral is directly proportional to the pulse repetition frequency. The collector 28 of the transistor 42 is the second input of the multiplier. For the operating voltage, a resistor 54, connected in the feed-back circuit 40, conducts the output voltage of the operational amplifier 22 to the collector of the transistor 42. Con sequently, the amplitude of the pulses on the collector 28 is dependent on the output signal of the operational amplifier 22. The voltage across the resistor 55 is therefore an analog signal which is proportional to the product of u, u

It is apparent from the explanation in connection with the preceding expression (1) that to this product must be added the signal u,,. This is done by the combination of resistors 55, 57, and 58. The resistor 58 is a trimming potentiometer in order to enable making the entire arrangement symmetrical. The condition of expression (1) is thus fulfilled.

As previously remarked, the time integral of the input signal u, must be formed in order that the transistor 42 can operate as a time-division multiplier. Likewise, the time integral of the input signal 11,, must also be formed in order that the entire stage can operate as a divider. For this reason, the capacitor 59 is connected in the feed-back circuit 40 to make the operational amplifier 22 an integrator. A setting resistor 60- connects the second input 31 of the operational amplifier to the lead 67. The setting resistor 60 enables the input resistance of the second input to be adapted to that of the first input 30. The voltage divider 51, 53 sets the emitter voltage of the two transistors 42 and 43 and the voltage at the second input 31 of the operational amplifier 22.

The electrohydraulic control 24 can be replaced by an electric measuring instrument that is connected be tween the output 32 and one of the two voltage rails 48 and 49. If a measuring instrument is connected into the circuit, the instrument indicates directly the quotient of The second embodiment, shown in FIG. 3, offers the further advantage of being able to adapt the output signal of the operational amplifier 22 exactly to the input level of the electrohydraulic control 24. To this end, a second operational amplifier 23 is provided. A resistor 70 connects the output signal of the first operational amplifier 22 to the first, inverting, input 80 of the second operational amplifier 23. A voltage divider, consisting of the resistors 71 and 72, fixes the voltage at the second, non-inverting, input 81 of the second operational amplifier.

In this embodiment, the output of the second operational amplifier, not the output of the first operational amplifier, is connected to the feed-back lead 40. The output of the second operational amplifier 23 is also connected by a resistor 76 to the positive rail 48 and by a resistor 75 to the inverting input 80. The terminals 78 and 79 are respectively connected to the positive rail 48 and the negative rail 49 to provide electric power for the amplifier 43. The negative feed-back resistor 75 determines the amplification factor of the amplifier 23. By choosing the correct value for the resistor 75, it is possible to adapt the output signal of the first operational amplifier 22 to the input level of the electrohydraulic control 24. An additional integrating capacitor 61, connected between the output and the second input 31 of the operational amplifier 22, further filters the direct current output voltage.

The two integrating capacitors 59 and 61 also act as low pass filters, so that the otherwise essential low pass filter can be omitted.

If the input signals u, and 14,, are small, the residual voltages of the transistors 42 and 43 can introduce noticeable errors into the measured quotient. This source of error is easily eliminated by inversely driving transistor 42 or both transistors 42 and 43, the collector and emitter connections being reversed from those shown in FIGS. 2 and 3. The emitter of the second transistor 42 is then connected to the junction 28, and the collector to the lead 67. Likewise, the collector of the third transistor 43 is connected to the lead 67, and the emitter to the junction between the resistors 56 and 57.

The pulse shapers 13 and 13' can be simplified somewhat by omitting the two Schmitt triggers l6 and 17. This is possible, for example, if the two amplifier stages 14 and sufficiently distort the approximately sin wave voltage output from the rpm. signal generators 11 and 12. Care must be taken to prevent the two amplifier stages 14 and 15 from delaying the return of the monostable multivibrators 18 and 19 to their stable states.

There is a single discrepancy between the block diagram shown in FIG. 1 and the circuit diagrams, shown in FIGS. 2 and 3, of the two embodiments. This discrepancy concerns the connections of the inputs of the operational amplifier 22. In FIG. 1, the output of the multiplier 21 is connected to the first input 30, and the output of the monostable multivibrator 19 is connected to the second input 31 of the operational amplifier 22. On the other hand, both of these outputs are connected by the summing resistors 55, 57, 58 to the first input 30 of the operational amplifier. The circuits shown in FIGS. 2 and 3 were chosen because they permit a higher measuring accuracy. If the improved accuracy is not required, the circuits shown in FIGS. 2 and 3 can be simplified, and brought into agreement with the block diagram of FIG. 1, by omitting the inverting stage (transistor 41) and by connecting the resistor 57 between the collector of transistor 43 and the second input 31 of the operational amplifier 22. In this case, there will also be required in the embodiment shown in FIG. 2 a second integrating capacitor 61 connected between the output 32 and the second input 31, so that very little is saved in the way of components.

In a simple way, the invention described insures that the plow lifting arrangement is so controlled that at all speeds of the tractor there is an optimum slip of the driven wheels.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of circuits, differing from the types described above.

While the invention has been illustrated and described as embodied in an electronic slip control, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge readily adapt it for various applications without omitting features, that from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

We claim:

1. In combination with a wheeled vehicle having a driven axle and an undriven axle, a slippage-monitoring arrangement comprising, in combination, a first signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the driven axle; a second signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the undriven axle; and divider means having an input connected to the output of said first signal generator and having another input connected to the output of said second signal generator and operative for generating a ratio signal indicative of the ratio of the rotational speeds. of said axles, wherein said divider means includes a multiplier and operational amplifier means, said multiplier and said operational amplifier means each including first and second inputs, the output of said multiplier being connected to the first input of said operational amplifier means and further including a feed-back connection connected from the output of said operational amplifier means to the second input of said multiplier.

2. The arrangement as defined in claim 1, wherein the output of the first signal generator is connected to the first input of said multiplier, and the output of the second signal generator is connected to the second input of said operational amplifier means.

3. The means as defined in claim 1, further including a capacitor connected between said output of the operational amplifier means and said first input of said operational amplifier means for providing negative feedback, low pass filtration, and integration.

4. The means as defined in claim 1, further including a positive rail and a negative rail; a voltage divider connected between the positive and negative rails, said voltage divider having a tap; a setting resistor connected between said tap and the second input of said operational amplifier means; and a first transistor having an input connected to the output of said second signal generator, and wherein said multiplier includes a second transistor, the emitter or the collector of said first transistor and the emitter or the collector of said second transistor being connected to said tap.

5. The means as defined in claim 8, wherein of said first and second transistors at least the collector of said second transistor is connected to said tap so that at least said second transistor is inversely driven so as to reduce errors caused by residual voltages of the transistor.

6. The arrangement as defined in claim 4, said vehicle being provided with a ground-penetrating implement and control means connected to said divider means and operative for varying the extent of penetration of said implement into the ground in dependence upon the value of said ratio signal, and wherein said operational amplifier means includes first and second operational amplifiers, said first operational amplifier being connected to said multiplier to receive as input the output of said multiplier, said second operational amplifier being connected to said first operational amplifier to receive as input the output of said first operational amplifier for obtaining a signal level suitable for said control means, and wherein said feedback connection is connected from the output of said second operational amplifier to said second input of said multiplier.

7. In combination with a wheeled vehicle having a driven axle and an undriven axle, a slippage-monitoring arrangement comprising, in combination, a first signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the driven axle; a

second signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the undriven axle; divider means. having an input connected to the output of said first signal generator and having another input connected to the output of said second signal generator and operative for generating a ratio signal indicative of the ratio of the rotational speeds of said axles, each of said signal generators having an output and said divider means having two inputs; and further including a first pulse shaper having an input connected to the output of said first generator and having an output connected to one input of said divider means, and a second pulse shaper having an input connected to the output of said second generator and having an input connected to the other input of said divider means, and wherein each of said pulse shapers comprises a respective monostable multivibrator, said vehicle being provided with a ground-penetrating implement and electrohydraulic control means connected to said divider means and operative for varying the extent of penetration of said implement into the ground in dependence upon the value of said ratio signal.

8. An arrangement as defined in claim 7, wherein said monostable multivibrator of said first pulse shaper includes a variable RC-timing network.

9. In a combination with a wheeled vehicle having a driven axle, an undriven axle, a ground-penetrating implement, and lifting means for raising and lowering said implement to vary the extent of penetration of said implement into the ground, a novel control arrangement comprising a first signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the driven axle; a second signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the undriven axle; divider means having an input connected to the output of said first signal generator and having another input connected to the output of said second signal generator and operative for generating a ratio signal indicative of the ratio of the rotational speeds of said axles; and means for causing said lifting means to vary the extent of penetration of said implement into the ground in such a manner as to maintain said ratio signal within a predetermined range. 

1. In combination with a wheeled vehicle having a driven axle and an undriven axle, a slippage-monitoring arrangement comprising, in combination, a first signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the driven axle; a second signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the undriven axle; and divider means having an input connected to the output of said first signal generator and having another input connected to the output of said second signal generator and operative for generating a ratio signal indicative of the ratio of the rotational speeds of said axles, wherein said divider means includes a multiplier and operational amplifier means, said multiplier and said operational amplifier means each including first and second inputs, the output of said multiplier being connected to the first input of said operational amplifier means and further including a feed-back connection connected from the output of said operational amplifier means to the second input of said multiplier.
 2. The arrangement as defined in claim 1, wherein the output of the first signal generator is connected to the first input of said multiplier, and the output of the second signal generator is connected to the second input of said operational amplifier means.
 3. The means as defined in claim 1, further including a capacitor connected between said output of the operational amplifier means and said first input of said operational amplifier means for providing negative feed-back, low pass filtration, and integration.
 4. The means as defined in claim 1, further including a positive rail and a negative rail; a voltage divider connected between the positive and negative rails, said voltage divider having a tap; a setting resistor connected between said tap and the second input of said operational amplifier means; and a first transistor having an input connected to the output of said second signal generator, and wherein said multiplier includes a second transistor, the emitter or the collector of said first transistor and the emitter or the collector of said second transistor being connected to said tap.
 5. The means as defined in claim 8, wherein of said first and second transistors at least the collector of said second transistor is connected to said tap so that at least said second transistor is inversely driven so as to reduce errors caused by residual voltages of the transistor.
 6. The arrangement as defined in claim 4, said vehicle being provided with a ground-penetrating implement and control means connected to said divider means and operative for varying the extent of penetration of said implement into the ground in dependence upon the value of said ratio signal, and wherein said operational amplifier means includes first and second operational amplifiers, said first operational amplifier being connected to said multiplier to receive as input the output of said multiplier, said second operational amplifier being connected to said first operational amplifier to receive as input the output oF said first operational amplifier for obtaining a signal level suitable for said control means, and wherein said feedback connection is connected from the output of said second operational amplifier to said second input of said multiplier.
 7. In combination with a wheeled vehicle having a driven axle and an undriven axle, a slippage-monitoring arrangement comprising, in combination, a first signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the driven axle; a second signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the undriven axle; divider means having an input connected to the output of said first signal generator and having another input connected to the output of said second signal generator and operative for generating a ratio signal indicative of the ratio of the rotational speeds of said axles, each of said signal generators having an output and said divider means having two inputs; and further including a first pulse shaper having an input connected to the output of said first generator and having an output connected to one input of said divider means, and a second pulse shaper having an input connected to the output of said second generator and having an input connected to the other input of said divider means, and wherein each of said pulse shapers comprises a respective monostable multivibrator, said vehicle being provided with a ground-penetrating implement and electrohydraulic control means connected to said divider means and operative for varying the extent of penetration of said implement into the ground in dependence upon the value of said ratio signal.
 8. An arrangement as defined in claim 7, wherein said monostable multivibrator of said first pulse shaper includes a variable RC-timing network.
 9. In a combination with a wheeled vehicle having a driven axle, an undriven axle, a ground-penetrating implement, and lifting means for raising and lowering said implement to vary the extent of penetration of said implement into the ground, a novel control arrangement comprising a first signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the driven axle; a second signal generator mounted on said vehicle and having an output, and operative for generating at its output a signal indicative of the rotational speed of the undriven axle; divider means having an input connected to the output of said first signal generator and having another input connected to the output of said second signal generator and operative for generating a ratio signal indicative of the ratio of the rotational speeds of said axles; and means for causing said lifting means to vary the extent of penetration of said implement into the ground in such a manner as to maintain said ratio signal within a predetermined range. 